1. Technical Field
This invention relates to the field of coordinate measuring machines. More specifically the invention relates to a guideway composite structure and method of manufacture for coordinate measuring machines.
2. Background Art
A coordinate measuring machine (CMM) is a device used to measure the dimensions of a work piece. Essentially, a CMM includes a probe attached to a mechanism capable of moving the probe in three dimensions and determining the location of the probe tip in space with respect to a three-dimensional coordinate system. By aligning the probe tip with multiple points on the surface of a work piece, the work piece can be mapped out within the coordinate system. As the need increases for machining or producing components within very tight tolerances, the need also increases for highly accurate and precise CMMs. Accuracy refers to the error in a measurement from the true value, while precision refers to the consistency with which a measurement can be made within a certain error range. If a CMM is limited in the accuracy or precision with which it can measure the dimensions of a work piece, then any resulting errors in measurement will carry through into a device manufactured using the measurements.
Several types of CMMs exist, each variation addresses ways to improve the accuracy or precision of measurements. Some of the patents describing such technologies include: Critelli, U.S. Pat. No. 5,581,012; Stott, U.S. Pat. No. 5,555,633; Shelton, U.S. Pat. No. 5,426,861; McMurtry, U.S. Pat. No. 5,402,981; Herzog, U.S. Pat. No. 5,396,712; Ogiwara, U.S. Pat. No. 5,339,531; Breyer et al., U.S. Pat. No. 5,333,386; Waeldele et al., U.S. Pat. No. 5,269,067; Raleigh et al., U.S. Pat. No. 5,257,461; Bury, U.S. Pat. No. 5,247,749; Schmitz et al., U.S. Pat. No. 4,932,136; and Hemmelgarn et al., U.S. Pat. Nos. 4,887,360 and 4,882,847, all of which are herein incorporated by reference for their pertinent and supportive teachings. However, conventional CMMs, such as those described in the references above, do not attain the accuracy and precision desired by today's users of CMMs.
One of the problems with conventional CMMs is that significant errors are introduced into measurements because of environmental temperature changes. There are essentially two related pathways through which temperature influences CMM measurements. First, most materials used in CMMs have a coefficient of thermal expansion (CTE) greater than 6.1.times.10.sup.-6 centimeter/centimeter/.degree. Celsius (3.4.times.10.sup.-6 inch/inch/.degree. Fahrenheit) such that environmental temperature changes cause significant expansion or contraction of the CMM structure to introduce noticeable errors into measurements. Second, even though the temperature is held constant while taking measurements, some materials in the CMM structure have a relatively large thermal inertia. Accordingly, if the materials have not yet stabilized at the environmental temperature, then, as the temperature of the CMM structure changes to reflect the environmental temperature, the structure will continue to expand or contract until equilibrium is established.
Attempts to use low CTE materials and decrease thermal inertia have not succeeded in attaining a CMM with the needed accuracy and precision. INVAR 36, an alloy of 64% iron and 36% nickel has a near zero CTE, however, it has only been used in minor components of CMMs such as measurement scales and reference samples. INVAR 36 has not been used in major structural portions of CMMs primarily because it has a relatively low modulus of elasticity (stiffness) of 141 GigaPascals (GPa) (20.5.times.10.sup.6 pounds per square inch (psi)). Accordingly, INVAR structures tend to flex and re-introduce errors that perhaps were otherwise reduced because of their low CTE.
Aluminum is widely used, even though it has a high CTE of 22.5.times.10.sup.-6 cm/cm/.degree. C., because it has a high thermal conductivity and is readily fabricated into complex geometries designed to stabilize the CMM structure. The high thermal conductivity reduces the problem of thermal inertia, and the complex geometries theoretically reduce errors from expansion and contraction. However, aluminum CMMs have proven incapable of achieving the accuracy and precision now in demand.
Ceramic, specifically alumina-ceramic (such as AD96), is used for some major CMM components, such as the cross beam guideway (bridge) in a bridge-style CMM. Ceramic has a high modulus of elasticity of 303 GPa (44.times.10.sup.6 psi), a CTE of 6.1.times.10.sup.-6 cm/cm/.degree. C., and, accordingly, provides a sufficiently stiff structure that is lightweight. However, 16-20 weeks are required to produce a final ceramic product, such structures are costly, and it is impossible to produce large monolithic structures of the size needed in typical CMMs. Instead, modular portions are produced and secured together, increasing cost. Also, ceramic is brittle and susceptible to cracking.
Granite is also used for a few major CMM components, however, granite has proven suitable only for a static base and rail or non-moving components. Granite may be finished by polishing and lapping techniques to produce excellent air bearing tracks. Unfortunately, granite is a poor choice for a moving guideway because it has a low modulus of elasticity of 26 to 86 GPa (3.8 to 12.5.times.10.sup.6 psi) and must have a large cross-section to attain sufficient stiffness and therefore additional mass. The resulting guideway is much heavier than desirable as a moving member in a CMM.
Thus, it can be seen from the above discussion that it would be an improvement in the art to provide a CMM structure with a low CTE, low thermal inertia, and a high modulus of elasticity that provides high quality bearing tracks, is relatively lightweight and is not limited in size or configuration, yet can be produced with a short lead-time at a moderate cost.